1. Field of the Invention
This invention relates to a magnetostatic wave device such as a microwave filter, a resonator, or an S/N (signal-to-noise ratio) enhancer.
2. Description of the Related Art
As an RF signal received by a satellite broadcasting television receiver weakens due to rain, snow (white specks) increases in reproduced pictures on a display of the television receiver. When the received RF signal falls into an unacceptable range, it is difficult to reproduce pictures on the display.
T. Nomoto et al have proposed an S/N enhancer for improving the signal-to-noise ratio of a received RF television signal (T. Nomoto et al., IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 8, August 1993, pp. 1316-1322). This prior-art S/N enhancer is of the cancel type, using two magnetostatic surface wave filters.
Y. Ishikawa et al have developed an adaptor for a satellite broadcasting television receiver (Y. Ishikawa et al., Proc. of 1994 Asia Pacific Conference, pp. 179-183). This prior-art adaptor uses an S/N enhancer designed to operate for a 1.9-GHz band. In the prior-art adaptor, a signal in a first IF band of 1 GHz to 1.3 GHz which is derived from a received RF signal is up-converted to a 1.9-GHz signal, and the 1.9-GHz signal is processed by the S/N enhancer. The processing-resultant signal, that is, the filtering-resultant signal, is down-converted back to a signal in the first IF band. The prior-art adaptor includes frequency converters in addition to the S/N enhancer. Thus, the prior-art adaptor tends to be expensive.
An S/N enhancer capable of operating at frequencies of 1 GHz to 1.3 GHz has been desired. Also, an S/N enhancer has been desired which can operate for a 400-MHz band, that is, a second IF band in a satellite broadcasting television receiver.
Japanese published unexamined patent application 7-130539 discloses a magnetostatic surface wave device. This prior-art magnetostatic surface wave device includes a film of single crystal of garnet which is grown on a Gd.sub.3 Ga.sub.5 O.sub.12 substrate. The Gd.sub.3 Ga.sub.5 O.sub.12 substrate has a plane orientation being one from among (110), (100), and (211). In the prior-art magnetostatic surface wave device, an anisotropic magnetic field can be weak so that the lower limit of the frequency band for the propagation of magnetostatic surface wave can be a relatively low frequency. In Japanese application 7-130539, the lowest frequency of magnetostatic surface wave is 900 MHz when a saturation magnetization is 1,760 G and the plane orientation of the substrate is (100). Accordingly, it is difficult to use the prior-art magnetostatic surface wave device as an S/N enhancer operating for a 400-MHz band.
T. Kuki et al have developed a reflection-type S/N enhancer operating for a 400-MHz band (T. Kuki et al., 1995 IEEE MTT-S Digest, pp. 111-114). To attain an operating frequency of 400 Hz, this prior-art reflection-type S/N enhancer uses a thin film of YIG which has a relatively low saturation magnetization. Since this prior-art S/N enhance is of the reflection type, the width of an operating frequency band thereof is a small value equal to 40 MHz. The operating frequency of this prior-art reflection-type S/N enhancer tends to considerably depend on an ambient temperature. Accordingly, it is difficult to practically use this prior-art reflection-type S/N enhancer.
T. Kuki et al. used a magnetostatic wave in a reflection-type S/N enhancer designed as a mixture of surface wave and backward volume wave (T. Kuki et al., Manuscript C-106, General Meeting 1996, Japanese Institute of Electronics, Information and Communication Engineers). This prior-art design is effective in lowering and widening the operating frequency band of the reflection-type S/N enhancer although a saturation magnetization is relatively great. The temperature characteristic of the reflection-type S/N enhancer of the prior-art design has not been investigated. To operate the reflection-type S/N enhancer of the prior-art design, it is necessary to further lower the operating frequency thereof.